This invention relates to a control device for regulating illumination in a controlled lighting area.
With the current interest in energy conservation, architects and designers are increasingly incorporating skylights and large window surfaces into buildings and other structures in order to take maximum advantage of natural light. However, because natural light can vary substantially, depending on weather conditions and attitude of the sun, it cannot be depended upon to provide sustained periods of adequate illumination, even during mid-day hours. Therefore, it has become necessary to supplement the natural light with artificial light during periods when the natural light falls below a predetermined level. This has been accomplished in the prior art by utilizing a control means comprising a light sensing device for measuring natural light coupled to a switch means to turn artificial lights on and off when the natural light reaches said predetermined level. However, weather conditions may be such that bright sunlight will shine intermittently through slits in the clouds, or passing clouds may continuously produce dark shadows alternating with bright sunlight. In such event, the fluctuations in the natural light will cause the artificial lights to switch on and off repeatedly, thereby causing concomitant changes in illumination which are distracting, as well as irritating to persons working in the controlled lighting area. Further, such excessive and unnecessary switching of the artificial lights may reduce the life of the fluorescent lights. This problem is inherent in prior art illumination control devices.
In addition to on and off oscillation caused by variations in natural light, illumination control devices may also oscillate if stray or reflected artificial light strikes the light sensing device. This usually occurs because the physical configuration of the artificial lights or objects within the controlled lighting area cause artificial light to be reflected so as to reach the light sensing device. Since the light sensing device cannot differentiate between natural and artificial light, it reacts as if the natural light alone has increased and consequently acts to turn off the artificial lights. With the source of the artificial light extinguished, the light sensing device will sense that the light is decreased and turn the lights back on again. This process will repeat itself indefinitely until such time as the intensity of the natural light becomes strong enough or weak enough to negate the effect of the stray artificial light.
The prior art has endeavored to solve this problem by circuit means which automatically create a differential between the amount of light needed to turn on the artificial lights and the amount of light needed to turn off the artificial lights. Thus, so long as this differential is greater than the amount of stray or reflected artificial light entering the light sensing device, the illumination control device will not oscillate. However, in the prior art, the turn on and turn off points must be raised or lowered in tandem, since the differential between such points is inherently fixed by the design of the circuit means. This is disadvantageous because controlled lighting areas have widely differing physical characteristics and configurations, and therefore, require correspondingly different differentials between turn on and turn off points in order to operate efficiently.